We have recently noticed variations in DE with the powders we have been spraying. We are using the same brand and type of powder and same spray conditions with the same gun. The only difference we see is the batch number from the powder manufacturer. What factors in the powder manufacturing process, particle composition, etc. can affect DE? We use exclusively agglomerated/sintered powders.

I don't have experience with HVOF. But with HVAF, Gordon is right. Particle size distribution is a big factor especially for the same supplier supposing that no other changes of production process. You should be careful of particles below 5 micron and over 30 micron. (I guess you are using 5-30 micron for HVAF).